The invention relates to a method for producing a nickel aluminide coating on a metal substrate, and to a part provided with such a coating.
The mechanical strength and oxidation resistance of materials used in the turbines of aircraft engines limit the performance of the engines. Recent prospective studies show that, for the turbine blades, for which the wall temperature currently reaches 1050-1100° C., the optimization of the compositions of the metal alloys used (nickel-based “superalloys”) and of the production methods, the improvement of the internal cooling circuits of the components and the use of thermal insulation coatings will not allow the intended wall temperatures of approximately 1300° C. to be reached. One method envisaged for operating at such temperatures is the use of composite materials constituted by two highly refractory phases, one of which is a metal phase M (where M is an Nb base alloyed with numerous elements such as Si, Ti, Cr, Hf, Al, etc.), which confers upon the material sufficient toughness at ambient temperature, and the other is an intermetallic phase M5Si3, which provides the desired strength and creep resistance at high temperature. These materials are called “materials of type Nb—Si” or “alloys of type Nb—Si” hereinbelow.
However, an obstacle to their development is their low oxidation resistance at high temperature, despite a large number of “favourable” elements being added to their initial composition (Si, Cr, B and Al). In fact, when such a material is subjected to the operating conditions of gas turbines, it is destroyed by oxidation within a period of between several minutes and about ten hours, depending on the grade used.
It appears that, in general, the oxygen penetrates into the metal phase and oxidizes it first, leaving the intermetallic phase M5Si3 virtually unaffected. It appears that the interfaces and also the grain boundaries assist the diffusion of oxygen.
Another problem is that, at low temperature, typically between 500 and 900° C., this type of material is incapable of rapidly developing a layer of protective oxides owing to a very low diffusion kinetics. As a result, the oxygen rapidly penetrates to the heart of the material, which makes it brittle. This type of oxidation is called the plague effect.
The main solutions which have been developed for protecting these materials of type Nb—Si are methods of diffusion coating by pack cementation, coatings obtained by siliconization, chromatization or aluminization or a combination of several such solutions.
According to Guo, X. P., Zhao, L. X., Guan, P., Kusabiraki, K. 2007 Materials Science Forum 561-565 (PART 1), pp. 371-374, it is possible to protect this type of material with a silicon-based coating deposited by pack cementation but with a halogenated activator. Xiaoxia Li and Chingen Zhou, 2007 Materials Science Forum 546-549 (PART 3), pp. 1721-1724 have applied siliconization by pack cementation activated by a halogenated activator to niobium silicide alloy coated with an MCrAlY deposited by plasma projection in air, the coating obtained by siliconization alone not being sufficiently protective.
Chen Chen et al. in Intermetallics, 15 (2007) 805-809 also propose protecting a material of type Nb—Si with a coating of silicon also comprising chromium. The chromium is deposited by pack cementation starting from chromium powder and a halogenated activator. The silicon is deposited either by pack cementation starting from silicon powders and a halogenated activator, or by molten salts.
Still starting from methods of pack cementation with a halogenated activator, Tian et al. propose a coating based on silicon and comprising either aluminium (Surface and Coating Technology, 203 (2009) 1161-1166) or yttrium (Surface and Coating Technology, 204 (2009) 313-318).
The major problem of all these techniques published in the literature within the public domain is that they use halogenated activators which, with the components of the niobium silicides, form highly reactive halogenated gases, which locally degrades the microstructure. It is therefore necessary to develop a technique which enables a coating to be created by diffusion without using halogenated gas.
Publication FR 2965568 describes a method for forming a protective coating against high-temperature oxidation on a surface of a refractory composite material based on silicon and niobium, wherein a non-halogenated gas comprising silicon and oxygen is reacted with chromium present on the surface to be protected in order to produce a composite coating having two phases, of which a first phase is an oxide phase based on silicon and having viscoplastic properties and a second phase is based on silicon, chromium and oxygen, and wherein said first and second phases are coalesced at high temperature, which allows a protective coating to be formed in which the second phase acts as a reservoir to reform, during operation, the first phase by reaction with an oxidizing gas.
The disadvantage of these types of method, whether the reactive gas is halogenated or not, is the necessity of using energy, these treatments being carried out in a furnace at high temperature. Another disadvantage for coatings based on silicon and chromium is their sensitivity to water vapour. As regards the NiAl coating obtained by the above-mentioned techniques, owing to the difference between the coefficients of expansion of the β-NiAl (15×10−6 K−1) and the material of type Nb—Si (10×10−6 K−1), the coating cracks in use.
In another field it has been verified that, owing to the strong chemical disparity, it is impossible to obtain by the current techniques of diffusion coating (pack cementation or chemical vapour deposition) a homogeneous coating of nickel aluminide on an assembly of materials, whether those materials, homogeneous or heterogeneous, are brazed, welded or simply screwed or riveted. By way of example, it is impossible, at present, to coat with a β-NiAl the assembly of a niobium-silicon alloy and a nickel-based superalloy, whatever the assembly technique used, least of all in a single operation. The other techniques of depositing an NiAl alloy, such as triode cathode sputtering (TCS) or physical vapour deposition (PVD), are directional and not suitable for complex shapes. Finally, chemical vapour deposition (CVD) permits only low deposition rates (approximately one micrometre per hour), which are not compatible with industrial production.
In addition, it is known that the reaction of synthesis of an NiAl material starting from Ni powder and Al powder is well known.
Accordingly, in the article “Review: reaction synthesis processing of Ni—Al intermetallic materials”, Materials Science and Engineering A 299 (2001) 1-15, K. Morsi mentions principally two methods which are carried out starting from nickel and aluminium powders. One of the methods, called “Self-propagating High temperature Synthesis” (abbreviated to SHS), carries out a self-propagating synthesis at high temperature. The other method employs a thermal blast or simultaneous combustion and can be described as a combustion method.
The articles of U. Anselmi-Tamburini and Z. A. Munir (The propagation of a solid-state combustion wave in Ni—Al foils in J. Appl. Phys., 1989, vol. 66, pp. 5039-45), of D. E. Alman, J. C. Rawers and J. A. Hawk (Microstructural and Failure Characteristics of Metal-Intermetallic Layered Sheet Composites in Metallurgical and Materials Transactions A, vol. 25A, 1995, 589ff) and of Ping Zhu, J. C. M. Li and C. T. Liu (Combustion reaction in multilayer nickel and aluminium foils, in Material Science and Engineering, A 239-240, 1997, 532-539) teach that it is also possible to synthesize intermetallic compounds, including Ni—Al, with alternate sheets of nickel and aluminium.
Publication FR 2752540 describes a method for applying an NiAl coating which is applied specifically to nickel- or cobalt-based superalloys. It is a method of the SHS type using compacted powders with application to the part (powders+alloy mixture) of a temperature gradient (200° C.), of a high pressure (hydrostatic pressure of the furnace up to 1.5 GPa) and a temperature of 1200° C.
Publication FR 2838753 describes an analogous method but for building up superalloys based on nickel or cobalt.
Another application of a coating, here NiAlPt, is described, still for nickel-based superalloys, by M. C. Record, H. de Jouvancourt and R. M. Marin-Ayral (Elaboration of Platinum-Modified NiAl Coatings by Combustion Synthesis: Simultaneous Repairing and Coating of Ni-based Superalloys, in International Journal of Self-Propagating High-Temperature Synthesis, 2007, Vol. 16, No. 4, pp. 199-206). This article relates to an SHS process with a brazing sheet introduced between the nickel-based superalloy and the compacted nickel-aluminium-platinum powder mixture, the whole being placed in a furnace with a temperature gradient and under high pressure.
All these techniques require a temperature gradient on the part and high pressures, which are not easy to employ especially for objects with thin walls which may be deformed under the effect of the pressure.
Publication US 2003/0211239 describes a method of aluminization by diffusion and teaches more particularly that, in order to make an NiAl coating, it is necessary to produce a composite electrolytic deposition comprising powders, one of which is optionally aluminium powder. This deposition must be baked above 871.11° C. (1600° F.), that is to say at a temperature higher than the melting point of pure aluminium. In the case where the quantity of aluminium is not sufficient, the remainder is supplied by a method of aluminization by diffusion.
Publication WO 2009/139833 describes a method for producing a barrier layer of aluminide, wherein the barrier layer comprises a nickel aluminide, an iron aluminide or a combination thereof, and the barrier layer is produced by a diffusion coating method on at least one surface of the article. It is, therefore, an aluminization method carried out at temperatures far higher than the melting point of pure aluminium, for example from 800° C. to 1200° C. and from 900° C. to 1100° C. The method also provides applying an aluminium foil to the surface of the substrate, and that annealing takes place at between 700 and 1200° C., therefore far above the melting temperature of aluminium. It is, therefore, a technique by reaction with liquid aluminium.